The process in this invention utilizes sorbents and catalysts that are generally of fluorite-type structure. The fluorite-type crystalline structure of the oxides used in the present invention is based on the fundamental CaF2 unit, in which the cation(s) has a face-centered-cubic (fcc) lattice, while the oxygen forms a cube case encapsulated inside the fcc lattice. The cation is or includes a rare earth metal, such as cerium, that creates “oxygen deficiencies” in the lattice by valance change, e.g. from 4+ to 3+. This typical structure has a unique advantage of creating a high percentage of oxygen vacancies without reconstructional phase transition in the cation sublattice even at temperatures higher than 1000° C., which is fundamental to the large oxygen transfer capacity of the oxides. The oxygen sublattice is independent, and the migration of an oxide ion does not require a substantial cooperative movement of any cation, because the valence variation of the rare earth (RE) cation, e.g., cerium cation, between valence 4+ and 3+ can easily accommodate oxygen vacancies. REO2 can be reduced reversibly to REO1.714, giving a large percentage release of oxygen.
One process to which the invention is directed is reforming of hydrocarbons to form hydrogen gas, and more particularly to producing hydrogen gas fractions that are sufficiently free of CO that they can be used in proton exchange membrane (PEM) fuel cells with little or no further purification. Traditional single step steam reforming is generally carried out at very elevated temperatures, typically ranging from 800° C. to 1700° C. These temperatures are required to effect good hydrogen production yields. Such elevated temperatures require very substantial energy input, reducing the overall efficiency of hydrogen production. Additionally, a reformer reactor will produce an initial syngas with carbon monoxide levels in excess of 20% by volume which requires further downstream processing that initially brings this down below 1% (water gas shift) followed by additional shift reaction and membrane purification to bring this impurity level below 30 ppm required for hydrogen that is to be used in conjunction with proton exchange membrane (PEM) technologies. If carbon monoxide levels exceed 30 ppm, the anode catalyst in the PEM fuel cell, which is often comprised of a platinum-based material, will be poisoned and will rapidly lose hydrogen oxidation activity. Additionally, if diesel fuel or heavy hydrocarbons are used as the fuel source, separate desulfurization steps are also required. This is because sulfur-bearing species generally poison Pt cathode and anode electrocatalyst materials in a PEM fuel cell. For this reason, many reforming catalysts must operate at 900° C. or higher so that sulfur may be completely removed from the catalyst surface. This increases energy costs and makes the use of nanomaterials difficult due to the high likelihood of sintering active materials at these temperatures. It is one general object of the present invention to reduce the energy demands of hydrogen production from hydrocarbons and to reduce subsequent purification steps that hinder production throughput.
Additionally, if diesel fuel or heavy hydrocarbons are used as the fuel source, separate desulfurization steps are also required. This is because sulfur-bearing species generally poison Pt-based electrocatalyst materials in a PEM fuel cell. One oxygen transfer process of the present invention is desulfurization of petroleum fuels containing high amounts of sulfur-bearing chemical species.
Most commonly in steam reforming, gas phase hydrocarbon with methane is the hydrocarbon of choice as the hydrogen source. It is desirable in some instances that efficient hydrogen production can be obtained from higher weight hydrocarbons, e.g., C6-C20 hydrocarbon mixtures. One particular aspect of the present invention is directed to a reactor that can efficiently produce hydrogen gas, substantially free of carbon monoxide, from a range of hydrocarbons including heavy fuels such as jet fuel, generally considered to have the average empirical formula C11H21. Heavier hydrocarbons are difficult to vaporize and decompose into smaller hydrocarbon fragments, making reforming difficult at temperatures below 900° C. (in addition to the reasons associated with sulfur tolerance described above). Herein are described systems and methods for effecting such “cracking” below 650° C. and reforming the resulting lower hydrocarbon downstream in a subsequent step in the same reactor. High yield conversions at temperatures of 650° C. or well below are made possible by the catalyst materials of the present invention.
C6-C20 fuels frequently contain significant levels of sulfur-bearing compounds, and it is a further object of the invention to produce hydrogen gas with not only very low carbon monoxide levels, but also low sulfur-containing impurities. One aspect of the invention is directed to a method for desulfurization (reduction or elimination of sulfur-containing compounds) of petroleum fluids. Catalyst materials in accordance with the invention are useful in desulfurization of petroleum fluids.
One aspect of the invention is use of oxygen transfer catalyst material to crack, desulfurize and reform C6-C20 hydrocarbons to produce PEM-grade or near PEM-grade hydrogen in a continuous process. The continuous process is conveniently performed in a flow-through reactor. This aspect of the invention addresses a specific requirement of military units that frequently are supplied with significant amounts of jet fuel. It is contemplated that isolated military units may use hydrogen gas, produced at a relatively proximal facility, in portable fuel cells for electrical generation and replacement of batteries. Hydrogen gas used in fuel cells produces a lower detectable signature of local energy generation than do combustion methods.
While it is one general object of the invention to provide an apparatus that will efficiently generate hydrogen gas from C6-C20 hydrocarbon mixtures, it is understood that such a reactor will also efficiently produce hydrogen from lower molecular weight hydrocarbons, including C1-C5 hydrocarbons. For example, the catalysts of the present invention produce hydrogen from methane at substantially lower temperatures than was heretofore possible or efficient.
Catalyst materials in accordance with the invention are useful in both a one-step method and a two-step method to prepare very pure hydrogen (low CO and S-bearing species levels) at low temperatures (650° C. or below). The one-step initial reforming reaction is described in equation (I) below:CxHy+H2O(cat)→xCO+(y/2+1)H2  (I)
In equation (I), carbon monoxide is generated and comprises 1-50 vol % of the effluent stream. Carbon monoxide, which must be removed from a PEM fuel cell stream, is generally treated in a separate water gas shift reactor downstream. In the current invention, carbon monoxide is either drastically reduced in a single reactor by adding additional catalyst of formula M(1)xM(2)1-xO2-z where M(1) is a transition metal (preferably a first row transition metal), M(2) is a rare earth metal, x is between about 0.01 and about 0.9, and z represents a degree of oxygen deficiency in said catalyst relative to M(1) plus M(2). This allows in situ water gas shift reactions to take place according to equation (II):CO+H2O(cat)→CO2+H2  (II)                Alternatively, a pure hydrogen product may be generated from these same catalysts at low temperature (650° C. or below) according to equations (III) and (IV):xCaHb+M(1)wM(2)1-wO2-z→axCO+xb/2H2+M(1)wM(2)1-wO2-z-xa  (III)M(1)wM(2)1-wO2-z-xa+xaH2O→xaH2+M(1)wM(2)1-wO2-z  (IV)In the first step (III), a syngas consisting of hydrogen and carbon monoxide is generated at temperatures below 650° C. which allows sequestration of the carbon monoxide impurity exclusively in this stream. Because the mixed oxide is reduced in this step and oxygen vacancies are created, reaction (IV) can take place in which these vacancies are replenished using oxygen from the water, generating hydrogen in the process.        
In order to reduce reforming temperatures, nanoparticulate catalyst materials are used in this invention in place of those with micron-sized grains. These include transition metal-substituted and rare earth metal-substituted rare earth fluorites of formula: M(1)xM(2)1-xO2-z as described above in respect to reactions (III) and (IV). In the invention, reforming and “cracking” processes, associated with reactions I-IV are enabled at temperatures at or below 650° C. using materials with surface area at least 15 m2/g, preferably with surface area greater than 50 m2/g. The use of the nanopowder in a reforming, cracking, or water gas shift reaction has shown enormous activity enhancements at temperatures below 650° C., even as low as 200° C. when compared to micron-sized powders with the same compositions (R. Pati et al, 41st Power Sources Conference, pp. 227-230, 14-17 Jun. 2004). While Applicants are not bound by theory, it appears that the increased surface area allows enhanced oxygen vacancy formation kinetics at these lower temperatures and thus enables reforming, shift and hydrocarbon “cracking” reactions. It is not clear whether this enhancement is the result of increased reaction rate from enhanced surface area alone, or whether there is an accompanying intrinsic surface chemistry change associated with higher surface energies or chemical functionalities.